Colloidal metal nanocatalysts to advance orange II hydrogenolysis tracked by a microplate reader

The thermal reduction method was applied to synthesize metal nanoparticles using poly(1-vinyl-2-pyrrolidone) as an organic stabilizer to control metal nanoparticle agglomeration. Colloidal metal nanoparticles, gold, palladium, and gold–palladium nanoparticles were synthesized, and UV–visible spectrophotometry and high-resolution transmission electron microscopy analyses were conducted to characterize them. The metal nanoparticle micrographs showed well-dispersed particles with an average size of 9.6 nm (Au), 15.4 nm (Pd), and 10.6 nm (AuPd). All the colloidal metal nanoparticles served as nanocatalysts to advance a reductive degradation of orange II in presence of borohydride ions. For a prompt screening of catalytic activity, the microplate reader system was considered at a fixed maximum absorbance wavelength of λ 489 nm respected by orange II. Excess borohydride ions were used to construct pseudo-first kinetic conditions. The Langmuir–Hinshelwood model allowed the finding of kinetic activity on the surface of metal nanoparticles. AuPd nanocatalyst interface exhibited low activation energy (5.38 kJ mol−1) compared to the one on Au (8.19 kJ mol−1) and Pd (7.23 kJ mol−1).


Introduction
Organic dyes are widely explored by numerous manufacturing industries for multiple reasons. Textiles, plastics, foods, and engineering manufacturing are the prevalent sectors subjected to huge consumption of organic dyes [1][2][3][4]. Environmental reports revealed the presence of organic dye discharged by industrial into the surface water. That posed a dangerous risk to the environment and ecosystem due to their toxicity, mutagenic, and carcinogenic effects [5]. With high physicochemical resistances, the natural degradation of organic dyes is tedious and challenging [6,7]. Azo dyes such as organic II constituted a class of organic dyes the most used in industry. Organic dyes discharge in surface water estimated at 15% should be prohibited to protect aquatic biota and human lives [8,9]. Waste management strategies must be conceptualized to limit organic pollutants discharge. An efficient degradation approach should be aligned with waste management protocols for eco-friendly discharge in surface water.
Catalytic processes have been largely executed to achieve the complete degradation of organic dyes. Redox catalytic, photocatalytic, and electrocatalytic processes consist of efficient approaches to degrade organic dyes [10][11][12]. The interests of metal nanoparticles as nanocatalysts highly increased owing to their exceptional physicochemical properties improving remarkably the catalytic activity. Either colloidal or supported metal nanoparticles as nanocatalysts exhibited remarkable catalytic activity compared to corresponding bulk materials [13][14][15]. Chemical reduction, thermal reduction, and deposition-precipitation approaches constituted the privilege synthetic approaches for the designing of metal nanoparticles [16][17][18]. A reaction rate seemed to undergo an increase for bimetallic nanocatalysts compared to monometallic nanocatalysts [19,20]. Charge transfer accounted for the advanced justification for the catalytic activity of bimetallic nanocatalysts [21,22].
Catalytic activity exhibited by metal nanocatalysts pertained to their dimension and geometry [23]. The catalytic investigation reported that the reaction rate proportionally increased with the dimension of the metal nanocatalyst [24]. However, metal nanoparticles suffered from a higher tendency of agglomeration leading to particle growth. That erased the exceptional catalytic activity recorded at nanoscales from metal nanoparticles. Organic stabilizers are applied to prevent the particles coalescence tendency of metal nanoparticles [25]. The organic stabilizer can be applied throughout the metal nanoparticle synthesis or after the synthesis. Poly(1vinyl-2-pyrrolidone) is one of the organic stabilizers evoked for the stability and dispersion of metal nanoparticles [26]. Somorjai research group reported platinum nanoparticles stabilized poly(1-vinyl-2-pyrrolidone) deposited on silica (SBA-15) as nanocatalysts to facilitate hydrogenolysis of ethane [27].
Surface-catalyzed activity on metal nanocatalysts throughout catalytic reaction constitutes an essential step towards the reaction rate. It has been staged as the ratedetermining step for several catalytic reactions. Adsorption isotherm models are developed to comprehend a surface-catalyzed process [28,29]. The Langmuir-Hinshelwood (LH) kinetic model received much attention in the description of the surface-catalytic activity on metal nanocatalysts. The LH kinetic model led to kinetic and thermodynamic activities on metal nanocatalysts, which clarified the catalytic activity on the surface of the catalyst. The LH kinetic model unveiled that molecular spilling over on platinum nanocatalyst deposited on mesoporous iron oxide material improved the reaction rate of methyl orange reduction in the presence of excess borohydride ions [10].
The current catalytic investigation discusses the thermal reduction synthesis of monometallic and bimetallic nanoparticles stabilized by poly(1-vinyl-2-pyrrolidone). Micrograph analysis using high-resolution transmission electron techniques served to characterize the metal nanoparticles. Gold, palladium, and gold-palladium nanoparticles were synthesized and characterized. All the metal nanoparticles served as metal nanocatalysts to advance a prompt purification of wastewater under reductive conditions. A benchmark reaction constructed with orange II (OII) and sodium borohydride was performed to evaluate the catalytic activity of the metal nanocatalysts. The microplate reader speeded the data collection at a fixed maximum absorbance wavelength of OII. The LH kinetic model was applied to fit the data to gain deep insight into the catalytic activity on the surface of metal nanocatalysts.

Synthesis of PVP-stabilized nanoparticles NPs
The thermal reduction approach was adapted to synthesize metal nanoparticles (NPs) following the polyol method reported by Dang et al. [30]. Typically, a predetermined amount of PVP was dissolved in ethylene glycol followed by metal salts addition. The salted ethylene glycol solution was heated for a decided time using a carousel TM multi-reactor connected to a reflux condenser (Radley Discovery Technologies). Into 5 mL of ethylene glycol solutionwas distinctively dissolved 0.98 g of palladium salt and 0.16 g of gold salt. PVP amount of 0.3 g and 0.5 g was added to palladium solution and gold solution, respectively. Also, 0.4 g of PVP dissolved in 5 mL of ethylene glycol, a respective mass of palladium and gold salts valued at 1 g and 0.2 g were added to make an alloy metallic mixture. All the salted solutions were subjected to heating under the string. For 30 min, palladium solution was heated at 120 °C under constant stirring. At 100 °C, gold solution pH was adjusted to 9 with 1 M NaOH. The gold solution was heated at 180 °C under constant stirring for 15 min. The bimetallic solution was allocated 3 h of stirring at 80 °C.

Characterization
Shimadzu UV-1800 spectrophotometer was used to observe the absorbance peaks corresponding to the NPs. Each NPs solution was transferred in a 3 mL quartz cuvette for spectrophotometry analysis. Copper grids served as sample holders for a high-resolution transmission electron micrograph (HR-TEM) analysis. Each NPs solution was dropped on a copper-grid and allowed to dry before micrograph capture. JOEL JEM-2100F electron microscope of 200 kV accelerating voltage served for NPs images captures. Fourier transmission infrared analysis was distinctively conducted for the organic stabilizer (PVP) and stabilized NPs (Pd-PVP, Au-PVP, and AuPd-PVP) using Shimadzu IRAffinity-1 spectroscopy.

Catalytic evaluation
BioTek Power XS2 microplate reader allowed the monitoring of the catalytic process at λ = 489 nm with respect to the maximum absorbance wavelength of the model compound. Reductive degradation of OII initiated by borohydride ions (BH 4 − ) was performed as a model reaction and 24 wells plate 3 mL capacity purchased from Becton Dickinson Labware served of reactors. Ionic mobility was controlled by a carbonate buffer constructed with an equimolar ratio of bicarbonate and carbonate ions (3 × 10 -4 M). Typically, each colloidal nanocatalyst, concentration range starting with zero to 3.

Synthesis of Pd-PVP, Au-PVP, and bimetallic AuPd-PVP NPs
Metal ion color shift indicated their reduction to zero-valence metal nanoparticles. Palladium NPs formation was assessed by the color shift from light brown to dark brown. A light-yellow solution of ionic gold turned into a wine-red solution referred to as gold NPs. No significant color shift was noticed for AuPd NPs preparation. AuPd NPs solution displayed dark brown color. Each colloidal NPs solution was subjected to UV-vis spectrophotometry screening. A wavelength peak of λ 519 nm highlighted the formation of gold NPs, while the λ 270 nm peak referred to palladium NPs formation. The bimetallic solution UV-vis spectrum indicated two peaks at λ 519 nm and λ 270 nm which highlighted gold and palladium NPs presence, respectively. The detected UV-vis spectra peaks supported the NPs formation and the displayed coloration remained stable for the entire catalytic works. UV-vis spectra are depicted in Fig. S1.
HR-TEM analysis made visible the geometry of the colloidal NPs and allowed the estimation of their dimension. The geometry insights unveiled that Au-PVP were rhombus particles, while Pd-PVP was structured as spherical particles. Rhombus and spherical geometries were visible for the bimetallic AuPd NPs supporting the presence of gold and palladium nanoparticles. Rhombus and spherical shapes for Pd and Au NPs were reported by Koczkur et al. using PVP as the organic stabilizer [25]. The micrographs highlighted the dispersion of the metal NPs that's critical for their application in catalysis as nanocatalysts. Using ImageJ software, the estimated average size of the colloidal NPs was 9.6 ± 2.0 nm for Au-PVP, 15.4 ± 2.0 nm for Pd-PVP, and 10.6 ± 2.0 nm for AuPd-PVP. Fig. 1a Considering the FTIR spectra depicts in Fig. S3. It is noteworthy that the stabilization of the metal nanoparticles by the PVP occurred through a steric effect. The FTIR vibration peaks highlighted the lone pair of electrons' contributions from the nitrogen or carbonyl oxygen of the PVP repeating unit to the hybrid orbitals of Au and Pd ions [31]. The absorption peak at υ 1642 cm −1 is deduced to be a C=O double bond of the carbonyl group present in Pyrrolidone ring [32]. The peaks at.υ 1453 cm −1 can be ascribed to C-N stretching vibration due to the presence of the CH 2 group peculiar to the pyrrole ring present in PVP [33]. The comparison of the three spectra also showed similar peaks at υ 1025 cm −1 and υ 1086 cm −1 , while the broad bands that appeared between υ 3600-2400 cm −1 which correspond to the hydroxy group [34], are also peculiar to all the spectra. A similar FTIR spectrum of PVP and NPs-PVP supported the steric effect as a stabilization way of PVP to the NPs.

Catalytic evaluation
The OII hydrogenolysis was well monitored using a UV-Visible spectrophotometry screening which displayed its maximum absorbance peak at λ 489 nm [35,36]. For a period, the OII max. absorbance degraded to indicate its hydrogenolysis. An isosbestic point led to a new absorbance peak at λ 246 nm corresponding to sulfanilic acid. Late on sulfanilic acid underwent a complete degradation as shown in Fig. 2a displaying UV-Visible spectrophotometry OII hydrogenolysis for Au-PVP nanocatalyst. Gas chromatography mass-spectroscopy analysis was broadly reported to describe the mass degradation of the OII alcoholic part named 2-amino-1-naphthol [37]. They applied extraction method using dichloromethane as the extracting A pseudo-first order condition was set with an excess of the reducing agent compared to the model compound [38,39]. The pseudo-first order equation appeared in Eq. 1. The exponential expression of Eq. 1 served to fit the catalytic data using Origin Pro software data analyzing and plotting (Eq. 2) [40]. No degradation of OII was detected in the absence of metal nanocatalysts. The uncatalyzed run reflected in Fig. 2b followed a horizontal development of the normalized maximum absorbance at λ 489 nm over time highlighting zero degradation of OII. For an evolved concentration of nanocatalysts, it reflected a drop-down of the normalized maximum absorbance at λ 489 nm over time. That indicated a catalytic degradation of OII, where the degradation rate increased proportionally with the nanocatalyst concentration. Fig. 2b displayed the course of normalized maximum absorbance at λ The observed reaction rate is presented by k obs .
[OII] stands for OII concentration, while a described the reaction rate valued as one. X (absorbance) is the normalized maximum absorbance of OII. t symbolized the length of catalytic degradation. A and C are the amplitude and endpoint of the surface-catalyzed process.
A plot of the observed reaction rate throughout OII degradation versus the nanocatalyst concentration presented a linear development. That pointed out the surfacecatalyzed activity on the surface of nanocatalyst as a catalytic pathway towards OII hydrogenolysis. A plot of observation rate versus nanocatalyst concentration appeared in Fig. 2c for AuPd-PVP. Catalytic effect plots for Au-PVP and.
Pd-PVP are displayed in Fig. S4. With a linear progression of the observed reaction displayed in Fig. 2c and Fig. S4, the catalytic process occurred in the kinetic domain for the nanocatalysts. A kinetic domain stipulated that the surface-catalyzed activity on the nanocatalyst interface consisted of the rate-determining step towards the hydrogenolysis degradation of OII by borohydride ions [41,42]. Nanocatalyst concentration pointed as star points in Fig. 2c, and Fig. S4 was selected from the nanocatalyst effect investigation for the rest of the kinetic studies.
Several catalytic approaches appeared in the literature to advance a fast removal of OII assisted by novel metal nanocatalysts. Among them list photocatalytic, redoxcatalytic, and electrocatalytic degradations. We conducted a comparative observation on the scale of the reaction rate in this report to some reported by others. Luo et al. conducted what they named one-step green synthesis to design bimetallic Fe/ Pd nanoparticles for OII degradation [43]. At ambient temperature, they reported a reaction rate scale of. 0.32 h −1 for the removal of OII. Wang et al. designed the supported iron-copper bimetallic nanocatalysts for oxidative degradation of OII in the presence of hydrogen peroxide [44]. The bimetallic nanocatalyst led to 0.47 min −1 degradation rate, while monometallic nanocatalysts resulted in low reaction rates.
(Cu = 0.34 min −1 and Fe = 0.08 min −1 ). With a reaction scale dimension of minute or hour, the current report stands as the fastest catalytic approach to achieving an effective removal of organic dye. The LH kinetic model was applied to describe the surface-catalyzed activity on the surface of nanocatalysts. The surface-catalyzed activity described by the LH kinetic model presents a nanocatalyst as a regular surface of active sites upon which substrates must adsorb to generate active species before engaging in chemical activity during a catalytic transformation [45,46]. With a fixed concentration of nanocatalysts, varied OII and borohydride ions were considered to investigate the surface-catalyzed process. The OII adsorption resulted in a decrease in reaction rate, (2) X (absorbance) = A • e −k obs •t + C while borohydride ion adsorption elevated the reaction rate. Those observations are depicted in Figs. 3a-d for AuPd-PVP, while for.
Au-PVP and Pd-PVP, they figured in Figs. S5a-S5d and S6a-S6d. A reaction rate increase initiated by borohydride ions adsorption indicated active species release (hydrogen species). A decrease in reaction rate recorded from OII adsorption indicated the active site's obstruction. We are in the competitive adsorption process where two adsorbates discussed accessibility on the nanocatalyst's active sites [47].
The pseudo-first order equation (Eq. 1) should be reconsidered to include the surface of nanocatalyst and surface coverage of substrates for a better comprehension of the surface-catalyzed activity on the surface of nanocatalysts. A revised Eq. 1 is expressed in the Eq. 2. Nanocatalyst interface upon which the substrates adsorbed is represented by S. Surface coverage of substrates is symbolized by θ OII and θ BH − 4 . The surface coverage allowed the description of thermodynamic parameters generated The constant n and m range from 0 to 1 referred to as Sips exponential indicating the state of the nanocatalyst interface [48]. For n and m values equaled or closed to unity, the nanocatalyst interface is described as a regular surface. A low value of n and m to unity showed the heterogeneity of the nanocatalyst surface. The kinetic data for AuPd-PVP fit using the LH kinetic equation depicted in Fig. 3a-d. Kinetic data fit for Au-PVP and Pd-PVP are presented in Figs. S5 and S6). The LH kinetic equation detailed the catalytic activities on the surface of nanocatalysts (Eq. 6). The fit parameters unveiled the kinetic and thermodynamic activities on the surface of nanocatalysts throughout catalytic reductive of OII. The fit parameters are given in Table 1 wherein are kinetic rate on the surface of nanocatalysts (k•S) and the substrate equilibrium constants (K OII and K BH − 4 ) at a given temperature. The surfacecatalyzed process on the interface of PVP stabilized metal nanocatalysts is illustrated in Scheme 1. The values of the Sips constant for the nanocatalysts are close to the unity indicating that the surface-catalyzed activity occurred on a regular interface.
Two other adsorption models were applied to establish that the surface-catalyzed activity is responsible for the OII hydrogenolysis. The Eley-Rideal model served to access that only the surface-catalyzed activity between the nanocatalyst interface and the adsorbed substrates governed the observed degradation of organic dye [49]. The Mars-van Krevelen model evaluated the probable participation of adsorbed oxygen molecules on the nanocatalyst interface since the catalytic experimentation was conducted in the open system [50]. The mathematical expression of the Eley-Rideal and Mars-van Krevelen kinetic models is in the supplementary information. The kinetic data depicted in Figs. 3a-d, S5a-S5d, and S6a-S6d were fitted for convergences. The fit plots are pictured in Figs. S7a-S7f, and. S8a-S8f based on Mars-van Krevelen, and Eley-Rideal). The resulting divergence fits showed that indeed the surface-catalyzed activity on the surface of nanocatalysts controlled the catalytic reductive of OII and the absence of molecular oxygen on the interface of nanocatalysts.
The Arrhenius and Eyring equations served to determine catalytic and thermodynamic activities from the reductive degradation of OII and on the surface of nanocatalysts [51,52]. The van't Hoff equation led to the determination of thermodynamic parameters link to substrates adsorption-desorption on the surface of nanocatalysts [53]. The catalytic activity on the surface of nanocatalyst was found superior compared to the one from reductive degradation. That was expected owing to the focus given to the catalytic activity on the nanocatalyst surface excluding the other catalytic activities [43,44]. Negative activation energy highlighted a fast chemical activity throughout OII degradation using Au-PVP nanocatalyst [53]. Gibb's free activation energy indicated that the reductive degradation of OII was not a spontaneous process. However, the catalytic activities on the surface of nanocatalysts consisted of a spontaneous process. The AuPd-PVP nanocatalyst exhibited a higher catalytic activity than Au-PVP and Pd-PVP. That can be explained by the combining effect of the metal nanoparticles which Scheme 1 OII hydrogenolysis facilitated by NPs-PVP nanocatalyst could be a charge-transfer or synergist effect [21,22]. Table 2 numbers the activation energy and thermodynamic parameters from reductive degradation and on the surface of nanocatalysts.

Conclusion
Poly(1-vinyl-2-pyrrolidone) served as an organic stabilizer through thermal reduction synthesis and successfully permitted the formation of well-dispersed and stable gold, palladium, and gold-palladium nanoparticles. Through reductive degradation of OII using an excess of borohydride ions as a model reaction for wastewater purification, all the metal nanocatalysts exhibited exceptional catalytic activities for prompt elimination of organic dye. Through a well-detailed kinetic description based on the Langmuir-Hinshelwood kinetic approach, we unveiled the catalytic activity on the surface of metal nanocatalysts throughout the reductive degradation. The bimetallic nanocatalyst exhibited a high catalytic activity compared to monometallic nanocatalysts. That should be explained by a potential charge transfer or synergist effect between gold nanoparticles and palladium nanoparticles. With a spontaneous process on the surface of nanocatalysts, the interface nanocatalyst-substrates were exceptional towards the removal of the organic dye.